Control device for internal combustion engine

ABSTRACT

An operating range boundary for switching a cam for driving an intake valve (drive cam) is changed in a direction of increasing an engine load if a target EGR rate is predicted to increase across the contour line shown in FIG.  3  during an acceleration operation. As can be seen from comparing FIG.  6  and FIG.  7,  a switching boundary in FIG.  7  is changed in a higher load direction than that in FIG.  6.  By changing to such a high load direction, a range in which a large cam is selected is enlarged. That is, switching of the drive cam from the large cam to a small cam is delayed. Therefore, it is possible to suppress deterioration of the combustion state in the cylinder.

CROSS-REFERENCE TO RELATED APPLICATION

The present disclosure claims priority under 35 U.S.C. § 119 to JapanesePatent Applications No. 2017-26422, filed on Feb. 15, 2017. The contentsof these applications are incorporated herein by reference in theirentirety.

TECHNICAL FIELD

The present disclosure relates to a control device for an internalcombustion engine.

BACKGROUND

JP 2013-72342 A discloses a control device for an internal combustionengine which is configured to control an engine in which a part ofexhaust gas as external EGR gas is recirculated from an exhaust systemto an intake system. In such a conventional control device, an openingdegree of an EGR valve is controlled based on a map defining arelationship between an operating range defined by engine speed andengine load and a target amount of external EGR gas (hereinafterreferred to as a “target EGR amount”). In the map, the operating rangesare partitioned by contour lines of the target EGR amount. According tothe map, the target EGR amount is set to a highest value in apartitioned range including a middle-engine-speed-and-middle-engine-loadrange, and decreases from this partitioned range toward a peripheralpartitioned range.

The target EGR amounts in the map are obtained by an experiment orsimulation performed in advance. According to the map, an actualexternal EGR amount (hereinafter also referred to as an “actual EGRamount”) can be maintained at an optimum value during a steady operationin which the engine operating state stays in a partitioned range havingan equal target EGR amount. On the other hand, the actual EGR amount islargely affected by time lag during a transition operation in which theengine operating state is transferred across the contour line of thetarget EGR amount. When the engine operating state is transferred from apartitioned range with large target EGR amount to a partitioned rangewith low target EGR amount, for example, the large influence by time lagcauses a period during which the actual EGR amount becomes excessivewith respect to the target EGR amount. Then, the combustion in thisexcessive period tends to deteriorate easily. Accordingly, acountermeasure against such deterioration of combustion during thetransition operation is needed.

The present disclosure addresses the above described problem, and anobject of the present disclosure is to suppress o occur deterioration incombustion when the engine operating range and the engine operatingstate is transferred from the partitioned range with high target amountto the partitioned range with low target amount,in a case where anopening degree of an EGR valve is controlled based on the map defining arelationship between the target amount of external EGR gas and.

A first aspect of the present disclosure is a control device for aninternal combustion engine which is configured to control an engine inwhich a part of exhaust gas as external EGR gas is recirculated from anexhaust system to an intake system,

wherein the control device comprising:

an EGR map the defines a relationship between an operating range definedby engine speed and engine load and a target value of external EGR rate,and has a predetermined partitioned range in which the target value isset to a highest value; and

an operating angle map that defines a relationship between the operatingrange and an operating angle of an intake cain for driving an intakevalve of the engine,

wherein the operating angle map being set so that

a large operating angle is selected in a first region including a regioncorresponding to the predetermined partitioned range, the largeoperating angle being capable of closing the intake valve in a firstcrank angle section, and

a small operating angle is selected in a second region in which theengine load is higher than that of the first region, the small operatingangle being capable of closing the intake valve in a second crank anglesection that is located nearer to a bottom dead center side than thefirst crank angle section,

wherein the control device is configured to:

select the operating angle in accordance with the operating angle mapwhen it is predicted that the engine operating state stays in apartitioned range having the equal target value in the EGR map; and

when it is predicted that the engine operating state is transferred froma partitioned range with the high target value to a partitioned rangewith the low target value in the EGR map, in a case where the engineoperating state is transferred in a direction of increasing the engineload, change a boundary between the first region and the second regionin a direction of increasing the engine load, and then select theoperating angle in accordance with the operating angle map.

A second aspect of the present disclosure is the control device for aninternal combustion engine according to the first aspect,

wherein the control device is further configured to, when changing theboundary in the direction of increasing the engine load, increase adegree of a change of the boundary as a change rate of an acceleratoropening degree of the engine becomes larger.

A third aspect of the present disclosure is the control combustionengine according to the first aspect,

wherein the control device is further configured to when changing theboundary in the direction of increasing the engine load:

calculate a time interval from a change point of the target value set inaccordance with the EGR map to an increase starting point of an actualexternal EGR rate; and

increase the degree of the change of the boundary as the time intervalis larger.

A fourth aspect of the present disclosure is the control device for aninternal combustion engine according to any one of the first to thirdaspects,

wherein the engine comprising a turbocharger including a compressor anda turbine, and

the external EGR gas is recirculated from a downstream side of theturbine to an upstream side of the compressor.

According to the first aspect, when it is predicted that the engineoperating state is transferred from a partitioned range with high targetvalue of the external EGR rate to a partitioned range with low targetvalue of the external EGR rate in the EGR map, in a case where theengine operating state is transferred in a direction of increasing theengine load, the boundary between the first region and the second regionis changed in a direction of increasing the engine load, and then theoperating angle can be selected in accordance with the operating anglemap. When the boundary is changed in the direction of increasing theengine load, the first region in which the large operating angle isselected is enlarged. Accordingly, it is possible to continue to selectthe large operating angle even in the period in which the smalloperating angle is originally selected. Here, when selecting the largeoperating angle, the turbulence in the cylinder is larger than in a caseof selecting the small operating angle. Therefore, according to thefirst aspect, it is possible to suppress deterioration of combustion inthe period during which the actual EGR rate becomes excessive withrespect to the target EGR rate.

According to the second aspect, the degree of the change of the boundaryin the direction of increasing the engine load can be changed as thechange rate of the accelerator opening degree becomes larger.Accordingly, it is possible to suppress deterioration of combustion inthe period during which the actual EGR rate becomes excessive inaccordance with the change rate of the accelerator opening degree.

According to the third aspect, the time lag from the change point of thetarget value of the external EGR rate set in accordance with the EGR mapto the increase starting point of the actual value of the external EGRrate is directly calculated, and the degree of the change of theboundary can be increased as the time lag is larger.

Therefore, it is possible to suppress deterioration of combustion in theperiod during which the actual EGR rate becomes excessive.

According to the fourth aspect, it is possible to suppress deteriorationof combustion of the engine provided with an LPL-EGR device in theperiod during which the actual EGR rate becomes excessive.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram illustrating a configuration example of a systemaccording to a first embodiment of the present disclosure;

FIG. 2 is an exemplary graph describing cam profiles of two types ofintake cams that are provided in the system according to the firstembodiment of the present disclosure;

FIG. 3 is an exemplary graph showing a relationship between an engineoperating range and a target EGR rate;

FIG. 4 is an exemplary graph showing a relationship between the engineoperating range and the cam for driving the intake valve;

FIG. 5 is an exemplary graph describing an intake valve closing timing;

FIG. 6 is an exemplary graph showing a change in the engine operatingstate during a transition operation (acceleration operation);

FIG. 7 is a graph describing a method of changing a switching boundaryin the first embodiment of the present disclosure;

FIG. 8 is time charts describing a relationship between transitions ofan accelerator opening degree, an engine load, and an external EGR rate,and a drive cam, respectively, when an operating point is transferredfrom PA to PB as shown in FIG. 6;

FIG. 9 is time charts describing transitions of the accelerator openingdegrees, the engine load and the external EGR rates during theacceleration operation, respectively;

FIG. 10 is a graph describing a method of adjusting a switching boundaryin a second embodiment of the present disclosure; and

FIG. 11 is a graph describing a method of adjusting the switchingboundary in a third embodiment of the present disclosure.

DESCRIPTION OF EMBODIMENTS

Hereinafter, embodiments of the present disclosure are described basedon the drawings. It is to be noted that common elements in each figureare designated by the same reference numerals, and duplicateddescription thereof are omitted herein. It is also to be noted that thefollowing embodiments do not limit the present disclosure.

First Embodiment

A first embodiment of the present disclosure is described with referenceto FIGS. 1 to 8.

[Description of System Configuration Example]

FIG. 1 is a diagram illustrating a configuration example of a systemaccording to the first embodiment of the present disclosure. The systemillustrated in FIG. 1 is a system for an internal combustion enginemounted in a vehicle. The system illustrated in FIG. 1 includes aninternal combustion engine 10 as a driving source. The internalcombustion engine 10 is a four-stroke reciprocating engine, and also anin-line three cylinder engine. It is to be noted that the number andarrangement of cylinders of the internal combustion engine 10 are notparticularly limited to the above-described number and arrangement. Eachcylinder of the interval combustion engine 10 communicates with anintake pipe 12 and an exhaust pipe 14.

An intake system of the internal combustion engine 10 is described. Anair cleaner 16 is attached in the vicinity of an inlet of the intakepipe 12. A compressor 18 a of a turbocharger 18 is provided downstreamof the air cleaner 16. The compressor 18 a is driven by rotation of aturbine 18 b that is provided in the exhaust pipe 14, to compress intakeair. An electronic control throttle valve 20 is provided downstream ofthe compressor 18 a. An intake manifold 22 that is connected to intakeports of each cylinder is provided downstream of the throttle valve 20.A water-cooled type intercooler 24 is incorporated in the intakemanifold 22. Intake air flowing in the intercooler 24 is cooled by heatexchange with cooling water flowing in a cooling pipe 26.

Next, an exhaust system of the internal combustion engine 10 isdescribed. The turbine 18 b of the turbocharger 18 is attached to theexhaust pipe 14. The turbine 18 b is connected to the compressor 18 a.The turbine 18 b is rotated by energy of exhaust gas flowing in theexhaust pipe 14. A bypass pipe 28 that bypasses the turbine 18 b isprovided in a middle of the exhaust pipe 14. A WGV (waste gate valve) 30is provided in the bypass pipe 28. The WGV 30 is opened when an exhaustpipe pressure (back pressure) on an upstream side of the turbine 18 b ishigher than a predetermined value. When the WGV 30 is opened, a part ofexhaust gas flowing in the upstream side of the turbine 18 b flows intothe downstream side of the turbine 18 b through the bypass pipe 28.Catalysts 32 and 34 for cleaning exhaust gas are provided in thedownstream side of the turbine 18 b.

Next, an EGR system for the internal combustion engine 10 is described.The internal combustion engine 10 includes an LPL-EGR (low pressureloop—EGR) device 36. The LPL-EGR device 36 includes an EGR pipe 38 thatconnects the exhaust pipe 14 between the catalysts 32 and 34, and theintake pipe 12 on the upstream side of the compressor 18 a. Awater-cooled type EGR cooler 40 is provided in the middle of the EGRpipe 38. Exhaust gas flowing in the EGR cooler 40 (i.e., external EGRgas) is cooled by heat exchange with cooling water flowing in a coolingpipe 42. An electronic control EGR valve 44 is provided on thedownstream side of the EGR cooler 40. A change of an opening degree ofthe EGR valve 44 causes a change of a flow amount of the external EGRgas that flows from the EGR pipe 38 into the intake pipe 12. When theopening degree of the EGR valve 44 becomes larger, an external EGR rateincreases.

Next, a valve system for the internal combustion engine 10 is described.FIG. 2 is an exemplary graph describing cam profiles (meaning at leastone of a lift amount and an operating angle, the same shall applyhereinafter) of two types of intake cams that are provided in the systemaccording to the first embodiment of the present disclosure. Asillustrated in FIG. 2, the system according to the first embodimentincludes a large cam and a small cam as the two types of intake cams.The small cam has an operating angle and a lift amount that are smallerthan those of the large cam. The large cam and the small cam are carriedon a camshaft that rotates in synchronization with a crankshaft. Twopair of large and small cams are carried on one cylinder because twointake valves are provided per cylinder. However, the number of intakevalves per cylinder in the present disclosure may be one, or three ormore. One of the intake earns is used as an intake cam for driving theintake valve (hereinafter also referred to as “drive cam”). The drivecam is switched between the large cam and the small cam by a switchingoperation of a switching mechanism.

The camshaft carrying the large cam and the small cam is provided with aVVT (variable valve timing mechanism). The VVT is a mechanism thatvaries a rotational phase difference of the camshaft with respect to thecrankshaft thereby to vary a valve opening characteristic of the intakevalve. The VVT includes a housing that is connected to the crankshaftthrough a timing chain or the like, and a vane body that is provided inthe housing and attached to an end portion of the camshaft. Hydraulicpressure is supplied into a hydraulic chamber partitioned by the housingand the vane body, to thereby enable the vane body to be relativelyrotated with respect to the housing, and further enable the rotationalphase difference of the camshaft with respect to the crankshaft to bevaried. The hydraulic pressure supplied to the VVT is controlled by ahydraulic pressure control valve provided in a hydraulic pressure supplyline. A system of the VVT is known, and a configuration of the system isnot limited in the present disclosure, and thus the further descriptionsof the VVT are omitted.

Returning to FIG. 1, the configuration example of the system iscontinuously described. The system illustrated in FIG. 1 includes an ECU(Electronic Control Unit) 50 as a control device. The ECU 50 includes aRAM (Random Access Memory), a ROM (Read Only Memory), a CPU(microprocessor) and the like. The ECU 50 takes in and processes signalsfrom various sensors mounted in a vehicle. The various sensors includean air flow meter 52, a crank angle sensor 54, an accelerator openingdegree sensor 56, and a supercharging pressure sensor 58. The air flowmeter 52 is provided in the vicinity of the air cleaner 16, and detectsan intake air amount. The crank angle sensor 54 outputs a signalaccording to a rotation angle of the crankshaft. The accelerator openingdegree sensor 56 detects a step-on amount of an accelerator pedal by adriver. The supercharging pressure sensor 58 detects an intake pipepressure (supercharging pressure) on the upstream side of the throttlevalve 20. The ECU 50 takes in and processes the signals from the varioussensors to operate various actuators in accordance with a predeterminedcontrol program. The various actuators include the above-describedthrottle valve 20 and WGV 30. The various actuators also include a VVT60 and a cam switching mechanism 62.

[Characteristic Control in First Embodiment]

FIG. 3 is an exemplary graph showing a relationship between an engineoperating range and a target EGR rate. The relationship in FIG. 3 iscreated based on a simulation performed in advance. It is to he notedthat the target EGR rate means a target value obtained by dividing anexternal EGR amount by an intake air amount, or can be also referred toas a value obtained by dividing the above-described target EGR amount bythe intake air amount. Among ranges partitioned by contour lines shownin FIG. 3, the target FOR rate is set to the highest value in thepartitioned range including a middle-engine-speed-and-middle-engine-loadrange. Thus, the external EGR rate is increased in themiddle-engine-speed-and-middle-engine-load range that is used withparticularly high frequency, to decrease an intake air temperature,thereby improving the heat efficiency. The target EGR rate is set to alower value in an operating range for which the frequency of use becomesrelatively lower. Specifically, the target EGR rate is set to a lowervalue in the partitioned ranges including a high engine load range and alow engine load range compared with a value in the partitioned rangesincluding a middle engine load range. Similarly, the target EGR rate isset to a lower value in the partitioned ranges including a high enginespeed range and a low engine speed range compared with a value in thepartitioned ranges including a middle engine speed range. In the firstembodiment, the relationship shown in FIG. 3 is stored in the ROM of theECU as a map, and an actual operating state is applied to the map tothereby control an opening degree of the EGR valve.

In the first embodiment, the engine is controlled by combining an intakevalve closing timing with the above-described target EGR rate. FIG. 4 isan exemplary graph showing a relationship between the engine operatingrange and the cam for driving the intake valve. As shown in FIG. 4, thelarge cam is selected in the middle-engine-speed-and-middle-engine-loadrange and the low-engine-speed-and-low-engine-load range, and the smallcam is selected in the high-engine-speed-and-high-engine-load range. Inthe first embodiment, the relationship shown in FIG. 4 is stored in theROM of the ECU as a map, and an actual operating state is applied to themap to thereby control the switching operation of the cam switchingmechanism.

FIG. 5 is an exemplary graph describing an intake valve closing timing.As shown in FIG. 5, when a drive cam is a large cam, the intake valve isclosed in a crank angle section CA₁ retarded with respect to a bottomdead center (ABDC=0). On the other hand, when a drive cam is a smallcam, the intake valve is early closed in a crank angle section CA₂including the bottom dead center. Widths of the crank angle sectionsCA₁, CA₂ as shown in FIG. 5 are provided to change the intake valveclosing timing by the VVT. However, when the large cam is selected asthe drive earn to increase the engine output, the crank angle sectionCA₁ is set so as to include a crank angle at which the suctionefficiency is maximized. On the other hand, when the small cam having asmall lift amount is selected as the drive earn, the crank angle sectionCA₂ is set so as not to include the crank angle at which the suctionefficiency is maximized. It is to be noted that the suction efficiencyshown in FIG. 5 can be obtained under operating conditions in which theengine speed is fixed, for example.

When the EGR valve is controlled based on the relationship shown in FIG.3, an actual external EGR rate (hereinafter also referred to as an“actual EGR rate”) can be controlled to an optimum value during a steadyoperation in which the engine operating state stays in a partitionedrange having an equal target EGR rate. On the other hand, the actual EGRrate is largely affected by time lag during a transition operation inwhich the engine operating state is transferred across the contour lineof the target FOR rate. This problem is described with reference to FIG.6. FIG. 6 is an exemplary graph showing a change in the engine operatingstate during the transition operation. FIG. 6 is a graph in which therelationships shown in FIG. 3 and FIG. 4 are combined. FIG. 6 shows anexample of a change in the engine operating state during an accelerationoperation. This example assumes that the engine operating state ischanged from an operating point PA to an operating point PB. When theoperating point is transferred from PA to PB, the operating point istransferred from a partition range R₁ through a partition range R₂ to apartitioned range R₃.

The target EGR rate is set to the highest value in the partitioned rangeR₁, and becomes lower in order of the partitioned ranges R₂, R₃, and R₄.Thus, when the operating point is transferred from PA to PB, the targetEGR rate continues to decrease. However, the time lag produces a periodduring which the actual EGR rate becomes excessive with respect to thetarget EGR rate. When such an excessive period of the actual EGR rateoccurs, combustion state in the cylinder tends to became unstable.Furthermore, when the drive cam is switched from the large cam to thesmall cam during the excessive period, turbulence in the cylinder isreduced. Therefore, deterioration of the combustion state in thecylinder cannot be avoided.

In consideration of these reasons, in the first embodiment, an operatingrange boundary for switching the drive cam (hereinafter also referred toas a “switching boundary”) is changed in a direction of increasing theengine load if the target EGR rate is predicted to increase across thecontour line shown in FIG. 3 during the acceleration operation. FIG. 7is a graph describing a method of changing the switching boundary in thefirst embodiment of the present disclosure. Operating points PA, PB andpartitioned ranges R₁ to R₄ that are shown in FIG. 7 correspond to theoperating points PA, PB and partitioned ranges R₁ to R₄ that are shownin FIG. 6, respectively. As can be seen from comparing FIG. 6 and FIG.7, a switching boundary in FIG. 7 is changed in a higher load directionthan that in FIG. 6. By changing to such a high load direction, a rangein Which the large cam is selected is enlarged. That is, switching ofthe drive cam from the large earn to the small cam is delayed.Therefore, it is possible to suppress deterioration of the combustionstate in the cylinder due to the switch of the drive cam during theexcessive period.

FIG. 8 is time charts describing a relationship between transitions ofan accelerator opening degree, an engine load, and an external EGR rate,and a drive, respectively, when the operating point is transferred fromPA to PB as shown in FIG. 6. As shown in FIG. 8, the accelerator openingdegree starts o increase at a time t₁. When the accelerator openingdegree increases, the engine load increases from LA to LB. It is to benoted that the engine load. LA shown in FIG. 8 represents the engineload at the operating point PA shown in FIG. 6, and the engine load LBrepresents the engine load at the operating point PB shown in FIG. 6.

Since the target EGR rate follows the relationship shown in FIG. 3, whenthe engine load increases after the time ti, the target EGR ratedecreases. When the target EGR rate decreases, the EGR valve openingdegree becomes smaller, and the actual EGR rate also decreases. Theactual EGR rate starts to increase after the time ti because this isaffected by the above-described time lag. It is to be noted that thetarget EGR rate starts to decrease after reaching the maximum valvebecause the operating point is transferred from the partitioned range R₁to the partitioned range R₂ as shown in FIG.

The ECU predicts whether the target EGR rate increases across thecontour line shown in FIG. 6 based on a change rate of the acceleratoropening degree after the time t₁. For example, the ECU stores in the ROMa threshold set in advance based on intervals between the contour linesof the target EGR to compare between the threshold and the change rateof the accelerator opening rate. When the ECU determines that the changerate of the accelerator opening degree exceeds the threshold after thetime t₁, the ECU predicts that the target EGR rate increases across thecontour line shown in FIG. 3.

In FIG. 8, it is predicted at the time t2 that the target EGR rateincreases across the contour line shown in FIG. 3. When the drive cam isswitched, at the time t₂, based on the relationship shown in FIG. 4 fromthe large cam to the small cam, the combustion state in the cylindereasily deteriorates. In this respect, in the first embodiment, since theswitching boundary is changed in the direction of increasing the engineload, it is possible to continue to select the large cam until a time t₃being later than the time t₂. Therefore, it is possible to suppress thedeterioration of the combustion state in the cylinder during theexcessive period.

In the above-described first embodiment, the map defining therelationship shown in FIG. 3 corresponds to an “EGR map” in a firstaspect. The map defining the relationship shown in FIG. 4 corresponds toan “operating angle map” of the first aspect. The partitioned range R₁shown in FIG. 6 corresponds to a “predetermined partitioned range” ofthe first aspect. The crank angle section CA₁ described in FIG. 5corresponds to a “first crank angle section” of the first aspect. Thecrank angle section CA₂ corresponds to a “second crank angle section” ofthe first aspect.

Second Embodiment

A second embodiment of the present disclosure is described withreference to FIGS. 9 to 10.

[Characteristic Control in Second Embodiment]

In the above-described first embodiment, when it is predicted during theacceleration operation that the target EGR rate increases across thecontour line shown in FIG. 3, the switching boundary is changed in thedirection of increasing the engine load. The second embodiment takes achange rate of an accelerator opening degree during the accelerationoperation into consideration.

FIG. 9 is time charts describing transitions of the accelerator openingdegrees, the engine load and the external EGR rates during theacceleration operation, respectively. FIG. 9 shows a case of rapidacceleration in which the rate of change in the accelerator openingdegree is large and a case of slow acceleration in which the rate ofchange in the accelerator opening degree is small. As shown in FIG. 9,in the case of rapid acceleration, the increase in the engine load isslower than that in the case of slow acceleration, and the decrease inthe target EGR rate is also slower than that in the case of slowacceleration.

In the second embodiment, therefore, a position of the switchingboundary is adjusted in accordance with the change rate of theaccelerator opening degree during the acceleration operation. FIG. 10 isa graph describing a method of adjusting the switching boundary in thesecond embodiment of the present disclosure. As shown in FIG. 10, in thesecond embodiment, the engine load at the switching boundary is set to ahigher value as the change rate becomes larger. That is, the degree ofthe change of the switching boundary is increased as the change ratebecomes larger. Therefore, it is possible to suppress the deteriorationof the combustion state in the cylinder during the excessive period inaccordance with the change rate of the accelerator opening degree. It isto be noted that the engine load value when the change rate of theaccelerator opening degree equals zero as shown in FIG. 10 correspondsto the engine load value at the switching boundary during the steadyoperation shown in FIG. 4.

Third Embodiment

A third embodiment of the present disclosure is described with referenceto FIG. 1.

[Characteristic Control in Third Embodiment]

In the first embodiment, the switching boundary is changed in thedirection of increasing the engine load when the target EGR rateincreases across the contour line shown in FIG. 3. As described above,this reason is because the time lag produces the period during which theactual EGR rate becomes excessive with respect to the target EGR rate.In the third embodiment, the ECU calculates the actual EGR rate duringthe acceleration operation to predict the time lag. The actual EGR rateis calculated based on an intake air amount, a supercharging pressureand an actual opening degree of the EGR valve, for example. The time lagcorresponds to a time interval from a change point of the target EGRrate during the acceleration operation to an increase starting point ofthe actual EGR rate. Therefore, the actual EGR rate during theacceleration operation is calculated to obtain the above-described timeinterval, and the actual time lag Δt can be thereby predicted.

In the third embodiment, a position of the switching boundary isadjusted based on the predicted time lag Δt. FIG. 11 is a graphdescribing a method of adjusting the switching boundary in the thirdembodiment of the present disclosure. As shown in FIG. 11, the degree ofthe change of the switching boundary is increased as the predicted timelag Δt becomes larger. That is, the engine load at the switchingboundary is set to a higher value as the predicted time lag Δt becomeslarger. Therefore, it is possible to suppress the deterioration of thecombustion state in the cylinder during the excessive period inaccordance with the time lag Δt. It is to be noted that the engine loadvalue when the time lag At equals zero as shown in FIG. 11 correspondsto the engine load value at the switching boundary during the steadyoperation shown in FIG. 4.

Other Embodiment

The above-described first to third embodiments are described on theassumption that the internal combustion engine is provided with anLPL-EGR device. However, the internal combustion engine may be providedwith an HPL-EGR (high pressure loop—EGR) device instead of the LPL-EGRdevice. The internal combustion engine may be provided with anon-supercharging EGR device instead of the LPL-EGR device. The internalcombustion engine may be provided with both of the LPL-EGR device andthe HPL-EGR device. However, considering that influence by the time lagof the external EGR rate becomes the largest in the LPL-EGR device, themethods described in the above-described first to third embodiments areparticularly effective in the internal combustion engine provided withthe LPL-EGR device.

What is claimed is:
 1. A control device for an internal combustionengine which is configured to control an engine in which a part ofexhaust gas as external EGR gas is recirculated from an exhaust systemto an intake system, wherein the control device comprising: an EGR mapthe defines a relationship between an operating range defined by enginespeed and engine load and a target value of external EGR rate, and has apredetermined partitioned range in which the target value is set to ahighest value; and an operating angle map that defines a relationshipbetween the operating range and an operating angle of an intake cam fordriving an intake valve of the engine, wherein the operating angle mapbeing set so that a large operating angle is selected in a first regionincluding a region corresponding to the predetermined partitioned range,the large operating angle being capable of closing the intake valve in afirst crank angle section, and a small operating angle is selected in asecond region in which the engine load is higher than that of the firstregion, the small operating angle being capable of closing the intakevalve in a second crank angle section that is located nearer to a bottomdead center side than the first crank angle section, wherein the controldevice is configured to: select the operating angle in accordance withthe operating angle map when it is predicted that the engine operatingstate stays in a partitioned range having the equal target value in theEGR map; and when it is predicted that the engine operating state istransferred from a partitioned range with the high target value to apartitioned range with the low target value in the EGR map, in a casewhere the engine operating state is transferred in a direction ofincreasing the engine load, change a boundary between the first regionand the second region in a direction of increasing the engine load, andthen select the operating angle in accordance with the operating anglemap.
 2. The control device for an internal combustion engine accordingto claim 1, wherein the control device is further configured to, whenchanging the boundary in the direction of increasing the engine load,increase a degree of a change of the boundary as a change rate of anaccelerator opening degree of the engine becomes larger.
 3. The controldevice for an internal combustion engine according to claim 1, whereinthe control device is further configured to when changing the boundaryin the direction of increasing the engine load: calculate a timeinterval from a change point of the target value set in accordance withthe EGR map to an increase starting point of an actual external EGRrate; and increase the degree of the change of the boundary as the timeinterval is larger.
 4. The control device for an internal combustionengine according to claim 1, wherein the engine comprising aturbocharger including a compressor and a turbine, and the external EGRgas is recirculated from a downstream side of the turbine to an upstreamside of the compressor.